The invention relates to the field of apparatus grinding or shredding material such as wood, refuse, paper etc.
Various apparatus are known for this purpose, generally comprising a plurality of rollers rotating in opposite directions, each made up of discs with rigid teeth, spaced out axially from one another on a drive shaft; the drive shafts are generally parallel and set at such a distance from each other as to allow the rollers to intersect. The teeth on the rollers of each axle are generally offset.
In particular EP 0319535 discloses a shredding apparatus with four rollers, namely two lower main rollers rotating in opposite directions, with their axles in a horizontal plane, and two outer side rollers, each cooperating with one of the main rollers. Beneath the rollers extends a grid or screen made up of perforated plates curved to the shape of a cylindrical surface, joined together and having as their axes the roller axes. This apparatus, though providing good results, requires a relatively high investment cost and has a high power consumption and in any case does not guarantee that there will not be occasional long pieces in the product leaving the shredder, especially in the case of wood.
Apparatus with two cooperating rollers, in which the shafts of the two rollers turn at different speeds, are also know to the art. In these apparatus it is common experience that, unless special measures are taken, a deposit of material forms between the disk type blades of the fastest shaft, and the slower shaft cannot clear this deposit. If left in place, the deposit builds-up, becomes compressed and hardens to the point of breaking the apparatus. The twin-shafted apparatus available on the market generally comprise cleaning sections fixed to the frame and protruding between the blades of the fast shaft, which remove the deposit as it forms. These cleaning sectors generally extend for about 90.degree. around the shaft of the relative roller. However, this arrangement does not allow a grille or grid to be used beneath the rollers since the material that is continually removed is pushed downward and, not always being of the right size to pass through the grille, it would accumulate against the grille itself to the point of breaking it.
A previous application by the same applicant, No. MI 93 A 001736, provides for a shredding apparatus with three roller of which the main two rotate in opposite directions at different speeds whilst a side roller cooperates with the faster of the pair of main rollers and rotates concordantly with it.
The inventor has noted that in the shredding apparatus outlined above it is possible for some relatively stiff shredded parts that have two dimensions smaller than the size of the holes in the grille to pass through said grille even if their third dimension is greater than the size of the holes in the grille, since said parts come to be disposed at right angles to the surface of the grille and are pushed through it in this position.